engine: introducing RefreshHostCapabilitiesCommand

RefreshHostCapabilitiesCommand will sync the backend with GetVdsCaps and
GetHardwareInfo data.

Today this sync is done just after few actions like activating a host.
The purpose of the new command is to allow performing this sync manually.
The command is allowed just if the host status is maintenance or up.

Change-Id: Iba25577a7b66f3acedc3b32b258560861d19c621
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>